In vitro and in vivo experimentation has shown that infection with murine sarcoma virus (Mo-MSV (MLV), resulted in rapid decreases in amounts of cytoplasmic ribonuclease activity which correlated inversely with levels of viral specific cytoplasmic mRNA and with concentrations of reverse transcriptase and p30 associated with the microsomal fractions. The significance of these changes are being studied and infections with murine leukemia virus (mo-MLV) are being examined to see if similar alterations occur.